The field of the invention comprises r.f. sputtering apparatus of the type which is intended primarily to sputter a coating of insulating photoconductive material upon a conductive substrate. The conductive substrate may comprise a synthetic resin web of material such as polyester or other polymer with an ohmic layer thereon or it may comprise a web of paper or of metal. The substrate is flexible to the extent that it can be moved through the apparatus at speed through the use of rollers and similar guiding, driving, tension control and direction changing means.
With respect to the background of the invention, it is clear that batch types of sputtering equipment and apparatus are limited in throughput. Where the supply and takeup reels must be located within the apparatus, after the total amount of substrate that can be handled within the apparatus has been coated, it is necessary to stop the operation of the machine or device and open the chamber so that the coated material can be removed. The supply reel must then be replenished; the end must be threaded through the sputtering electrodes and over the rollers and guide means and connected to the takeup mechanism; the chamber must be closed and sealed; the atmosphere must be pumped down; the background gas must be started up and adjusted for proper pressure; the electrical system must be connected and tuned; the parameters such as temperature, pressure, etc., must be adjusted for steady state conditions; the drive mechanism for the movement of the substrate must be started up; and perhaps many other functions must be performed before the coating can start. At that, there is no assurance that the properties of the resulting coated substrate will be the same as the coated substrate which resulted from the previous run or the same as the coated substrate that will result from the following run.
There is considerable loss of time in this procedure which has been described and which results in no product until steady state is reached. The invention herein is concerned with the commercial production of substrate which has been coated with insulating photoconductive material of the type which is disclosed in some detail in U.S. Pat. No. 4,025,339. The more familiar type of sputtering apparatus which uses bell jars and small chambers and which produces small several square centimeter rectangles or discs of sputtered material for each hour of use are of no value in achieving a throughput which can supply users with large quantities of electrophotographic material.
Several types of apparatus which are of the batch variety are disclosed in U.S. Pat. Nos. 3,829,373; 3,884,787; 3,905,887; 4,013,539; 4,014,779; and 4,026,787. Most of these patents include in their disclosures practical devices which are constructed in such a manner as to contain the supply and takeup reels and are thus strictly batch process apparatus. It is true that large reels of substrate can be coated in these devices, but again the need for stopping the process when the entire supply reel has been used up is expensive, time consuming and annoying. In U.S. Pat. Nos. 3,829,373; 3,884,787; 3,905,887; 4,013,539; and 4,026,787 continuous process coating apparatus is disclosed, but in every instance the substrate is led around a drum on the interior of the sputtering chamber. In an effort to obtain more than one turn around the drum, complicated mechanisms are suggested for shifting the turns of the substrate as it loops around the drum, but this must be limited to narrow strips and is well-nigh impossible for coating wide webs of substrate of the order of a meter and more in width.
It has been found that r.f. electrodes are required to be spaced sufficiently apart to avoid interference. This factor either would limit the number of electrodes or would dictate the provision of a huge drum.
Coating a web of substrate in a chamber which requires that the substrate move once around a drum has considerable limitations in the throughput even assuming that the substrate enters and leaves by way of air locks so that the process is continuous rather than batch-type in operation. Since the drum is normally the anode, the size of the drum for practical purposes is limited because its diameter will dictate to a large extent the size of the chamber and the apparatus itself. The largest known drums which are used in batch processes are of the order of a meter or so in diameter and perhaps a meter or so in axial length. The plasma which is developed between the targets which face the circumference of the drum is therefore limited by the amount of target area which can be disposed around the drum. For even the largest known drums, the target area is quite limited and hence the speed of movement of the substrate is also limited.
Given a drum which is about a meter in diameter, the application of a coating of cadmium sulfide of about 4000 A thick by means of twelve targets along the circumference of the drum can be effected in one pass at a speed not exceeding about 3 or 4 millimeters per second. To increase the throughput would require enormous drums with a substantial increase in the floor space, expense, problems of pump-down and introduction of background gas, problems of power transfer, etc.
In the case of substrates which require multiple coatings of different materials, as for example, that electrophotographic material disclosed in said U.S. Pat. No. 4,025,339 which is based upon a polyester substrate having a coating of indium-tin oxide of about 200 A thick on the substrate covered with a second coating of cadmium sulfide about 3000 A or more thick, it is difficult to produce the same in a single pass in apparatus which has a drum anode. This is true whether the apparatus is of batch type or continuous. The reason is that the space available along which to effect the principal coating is limited so that there is insufficient circumferential distance to provide for two or more different materials to be sputtered in the single pass around the drum.
Another problem which arises with an attempt to coat more than one material in a single pass in a single chamber is that contamination results. It is impossible fully to confine the sputtering plasma; hence if one would sputter a conductor like indium-tin oxide at one or two of the first reached targets and then sputter an insulator like cadmium sulfide in the remaining targets there is certain to be cross contamination. The stoichiometry of the photoconductor must be well-nigh perfect for the characteristics which are needed for proper charging, imaging, toning, etc. Adding even the most minute quantity of a conductor will destroy the stoichiometry.
In certain processes, before the ohmic layer of indium-tin oxide is applied, there is a flash of the photoconductive material of perhaps 30 A or so applied in the sputtering apparatus. In applying this thin layer, the substrate is moved through the plasma clouds at very high relative speed compared with the speed used to apply the main coating of photoconductive material.
As a result of these problems, it is common practice for the production of an electrophotographic member comprising a web of polyester with a photoconductive coating to pass the reel or spool of material through a sputtering machine three times. Each time a separate coating or layer is applied, the first being the flash of cadmium sulfide for example, the second being the ohmic layer and the third being the photoconductive coating. This requires handling the material three times and besides the problems of opening the macines to remove and install the spools or reels, in handling the material in air there is contamination because of moisture and impurities in the atmosphere of the room where the apparatus is located. Purifying the air and conditioning it to obviate this contamination poses additional problems and expense.
The invention solves all of the problems which have been described and also engenders additional advantages which will become apparent as the description proceeds. Whether the substrate requires three coatings as in the case of an insulating web of polyester or one or two as in the case of a web of tin plated steel or stainless steel, everything can be effected in a single pass and continuously without shutting down the apparatus until hundreds or even thousands of meters or substrate have passed through. The conditions under which the sputtering is carried on are uniform and maintained constant; any number of processes can be performed on the substrate while it is moving through the apparatus; the thickness of the coating can be controlled either by the speed of the substrate or the length of the apparatus. In the latter case, the apparatus can be made out of uniform and easily fabricated modules that are connected together by air locks. Certain types of apparatus which are commercially available for handling webs, as for example, tension controlling means and drive means can be used for moving the web through the apparatus and guiding and controlling it. The speed of sputtering is only limited by the length of the apparatus and can be substantially more than ten millimeters per second.
Prior art which is known to the applicant in addition to the above mentioned patents comprises the following U.S. Pat. Nos.:
2,965,067, Amelotte et al. PA1 2,989,026, Gardner et al. PA1 3,294,670, Charschan et al. PA1 4,009,090, Veigel
Applicant does not represent that this is a comprehensive list of the prior art; that the art represented by these patents is analogous to the invention or the field of the invention herein; or that the explanation of the patents is to infer that there is any pertinence thereto.
Amelotte et al. is for a vacuum coating apparatus in which a substrate moves between chambers of different pressure.
Gardner et al. discloses another continuous vacuum coating apparatus.
Charschan et al discloses apparatus and a method for sputtering conductive material at d.c. from a vertical cathode that is not shielded onto a sequence of independent rigid substrate members in carriers that are pushed along a path which carries them through a series of low pressure chambers. No r.f. is involved; no insulating material is sputtered; there is no continuous web of substrate material; the substrate members are not flexible; the system is impractical for use with a wide web of substrate material which must be sputtered at high speed.
Veigel teaches a sputter-coating process for putting a coating on glass sheets which are individual but move in a continuous line through a plurality of chambers.
The process and apparatus of the invention is different from any of these disclosures because the extremely difficult problems which are inherent in sputtering photoconductive coatings are in no way involved in these references and certainly are not solved thereby. Vacuum deposit is a totally different field with different requirements and different apparatus. The Veigel patent sputters conductive materials by means of d.c. power which will not operate to sputter insulating materials such as cadmium sulfide.